[FLINK-39490][web] Improve feedback when cluster is unreachable

[spuru9]

What is the purpose of the change

When the Flink cluster becomes unreachable (JobManager down, network blip, proxy returning 5xx), the dashboard gives no clear signal — pages sit with stale data and scattered per-request errors. This change detects persistent unreachability and surfaces a single, clear notification.

Brief change log

• Track consecutive network failures (status 0 / >=500) in StatusService and show a single persistent "Network Error" toast once 5 consecutive failures occur. The toast auto-dismisses on the next successful response and re-triggers after another streak if the user dismissed it during an ongoing outage.
• Minor fixes in the same area: JobService.loadJob now returns EMPTY on error instead of breaking the Job Detail view, and the header badge refreshes immediately when a new server error arrives.

Verifying this change

Verified manually: kill the JobManager and confirm the toast appears once after ~5 failed polls; restart and confirm it auto-dismisses; dismiss manually during an outage and confirm it reappears after another 5 failures.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

[flinkbot]

CI report:

• 463822d Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[spuru9]

@och5351 Can you check this too if possible.